FIG. 3 is a cross-sectional view showing a prior art GaAs MIS FET disclosed in "IEEE Electron Device Letters, Vol. EDL-7, No. 11". In FIG. 3, reference numeral 1 designates a semi-insulating GaAs substrate. A GaAs buffer layer 2 is disposed on the substrate 1. An n type GaAs channel layer 3 is disposed on the buffer layer 2. An undoped GaAlAs layer 4 is disposed on the channel layer 3. N.sup.+ type GaAs contact layers 5 are disposed on the GaAlAs layer 4. Source and drain ohmic electrodes 6 are disposed on the contact layers 5, respectively, and a gate Schottky barrier gate electrode 7 is disposed on GaAlAs layer 4.
A method of producing this GaAs MIS FET will be described hereinafter.
FIG. 4(a) is a cross-sectional view showing a wafer for the GaAs MIS FET produced by MBE. In FIG. 4(a), the same reference numerals as those shown in FIG. 3 designate the same or corresponding portions. The n type GaAs layer 3 functions as a channel of the FET and the undoped GaAsAs layer 4 functions as a gate insulating film.
After producing a wafer as shown in FIG. 4(a), source and drain ohmic electrodes 6 are produced on n.sup.+ type GaAs contact layer 5 as shown in FIG. 4(b) by using the lift-off method and an annealing. Thereafter, n.sup.+ GaAs contact layer 5 is selectively etched using photoresist as a mask to produce a recess structure, whereby the surface of GaAlAs layer 4 is exposed. Subsequently, Schottky Barrier gate electrode 7 is produced on GaAlAs layer 4 as shown in FIG. 4(c) using the photoresist pattern in the lift-off method.
In the prior art GaAs MIS FET constructed as described above, a high resistance GaAlAs layer is disposed between the n.sup.+ type GaAs contact layer 5 directly below the source and drain ohmic electrodes 6, and the n type GaAs layer 3 which functions as a channel layer. Therefore, the FET source resistance Rs and the drain resistance Rd are high and the high frequency characteristics are deteriorated.
In addition, in the selective etching of the n.sup.+ type GaAs layer 5 shown in FIG. 4(c), the GaAlAs layer 4 is also etched unfavorably, and this results in variations in element characteristics.